Pre-interpretation in JS

1. What is pre-interpretation?

In the current scope, before the JS code executes, it first takes the Var/function keyword (declare) or definition (defined).

2. How does the pre-interpretation work?

A. With the Var keyword, only the advance declaration is completed in the pre-interpretation phase; B. The pre-interpretation phase with the Function keyword completes the declaration and defines two operations.

3, pre-explained the point of attention?

A. The contents of the judging body should be pre-explained regardless of whether the condition is established or not;

For example: if (! (' A ' in window)} {

var a=12;

　　　　　　　　　　Determine if ' A ' is a property name for window, because the pre-interpretation of the declaration of ' a ', so true, take the reverse to false, the condition is not established, a=12 execution, output a=undefined;

}

B. Pre-interpretation of the "=" equal to the left side of the variable is pre-interpretation, the right represents the value, is not pre-interpretation;

Example 1:FN (); //Can be executed in front of the function, since the FN declaration and definition are complete at the time of the pre-interpretation

function fn () {

Console.log (' OK ');

}

Example 2:FN (); //This will cause an error in the FN is not a function, because at the time of the pre-interpretation is only the left side of the equal value is pre-explained, when the value of FN is undefined, not a function, cannot execute　

var fn=function () {

Console.log (' OK ');

}

3, in the global scope of pre-interpretation, the function in the self-executing functions is not involved, the contemporary execution to the corresponding region, declaration, definition, execution together complete;

4, although the function body returns the following code is not executed, but need to do the private scope of the pre-interpretation, and return is the back of the value, is not pre-interpretation;

　　For example: Var a=1;

function fn () {

Console.log (a); //Private scope pre-interpretation, first in the private scope Var A, stating that A is a private variable, but return the following code does not execute, so a is undefined;

return function () {}; //Return is not pre-interpreted and returns the corresponding address of the function;

var a=10;

}

FN ();

5, in the pre-interpretation of the time, if the name of a conflict, there is no need to re-declare, but need to re-assign value. (in JS, whether it is a variable or a function, as long as the name is the same, it is conflicting, JS in a name represents a variable, but the stored value can be any data type)

　　In the global scope, whether declaring a variable or defining a function, the property name is added to the window.

For example: FN (); //Output result 2

function fn () {console.log (1);}

FN (); //Output result 2

var fn=13;

FN (); //Error

function fn () {console.log (2);}

FN (); //Output result 2

Pre-interpretation in JS